fix: add missing `c8` dev dependency

[samialdury]

Fixes https://github.com/fastify/fastify-cli/issues/702

Checklist

• [x] run npm run test and npm run benchmark
• [x] tests and/or benchmarks are included
• [ ] documentation is changed or added
• [x] commit message and code follows the Developer's Certification of Origin and the Code of conduct

[mcollina]

Can you fix linting?